There are numerous applications where it is necessary or desirable to disconnect one or more loads from one power supply and to connect the load(s) to another power supply with minimum disruption in the load current(s). In aircraft and aerospace applications, a load transfer may be required between power sources external to the aircraft and on-board the aircraft or between separate on-board power sources. Prior on-board power sources are typically of the constant speed type having a hydromechanical constant speed drive which is coupled to the engine of the aircraft and which converts the variable speed motive power produced by the engine into constant speed motive power. A synchronous generator is coupled to the output of the constant speed drive and converts the constant speed motive power into constant frequency AC power for the loads.
More recently, attempts have been made at designing a practical power source which converts variable speed motive power into constant frequency electrical power without the use of hydromechanical components in the constant speed drive. For example, Dishner et al. U.S. Pat. No. 4,695,776 (Sundstrand) and Borger U.S. Pat. No. 4,572,961 disclose electrically compensated constant speed drives which include a mechanical differential having a first input coupled to the output of the prime mover and an output coupled to a synchronous generator. A speed compensation link in the form of first and second permanent magnet machines interconnected by a power converter is coupled between the prime mover and a second input of the differential. The power flow between the permanent magnet generators is controlled so that the required compensating speed is provided to the second differential input shaft to maintain the output shaft at a constant speed.
An alternative to the foregoing sources that does not use a constant speed drive is referred to as a variable speed, constant frequency (VSCF) generating system which includes a synchronous generator coupled directly to the aircraft engine and a power converter which converts the variable frequency output of the generator into constant frequency power for the loads. In multiple engine aircraft, each engine typically drives a separate VSCF system (sometimes referred to as a "channel") and the system outputs are coupled to a load bus through contactors. Interest in VSCF systems has increased of late owing to the push to design "all electric" aircraft in which the use of mechanical, hydromechanical and hydraulic components is minimized.
In a multiple-channel VSCF system of the above type, it may be necessary to change the source of power to the load bus from one channel to another or between an external AC source, such as a ground power cart, and one of the channels.
Corey U.S. Pat. No. 3,932,764 discloses a method and apparatus for transferring an electrical load from a utility AC power source to an inverter. The utility AC power source and the inverter are momentarily connected in parallel across the load and the inverter output voltage and phase are controlled to null the current provided by the AC source. Thereafter, the AC source is disconnected from the load to complete the power transfer